I've run AutoCAD since the days of the IBM PC. Right now I run AutoCAD and Revit on a dual core E8500 at 3.8GHz, 4GB ram, 32-bit WinXP, ATI FireGL. I have had a few files from clients that Revit wouldn't load with only 4GB. Our best 3 workstations are ones that I built using i5-750 at 3.8GHz, 8GB ram, 64-bit Win7, ATI FireGL. I also run AutoCAD and Revit on my Dell E6500 laptop- P9500 dual core at 2.53GHz, 4GB, 32-bit Vista. It runs OK but is a bit slow.
If I built a new workstation now I would probably use a 2500k, 16GB of ram, 64-bit Win7 and Nvidia Quadro. The 2500k can go to at least 4.5GHz. The 2600k is no real advantage for the additional $100 except that it might overclock 5% more.
There is a lot of talk about video cards. Autocad and Revit don't require much video card speed, but do require good drivers especially if you run a 64-bit OS. A card that will work with a 32-bit OS might not work with a 64-bit OS. On a 32-bit OS you can run them with just about anything that will drive a monitor. Autodesk's website has a database of video cards that they have tested and I would recommend checking this if you are serious about 64-bit software. I don't think there are any gamer cards in the database, and I know that you can run OK with an ATI FireGL or Nvidia Quadro in the under $150 price range.
But AutoCAD and Revit both will make use of as much CPU speed as you can get, and holler for more. To some extent the 32 bit versions also want fast disk access but the 64bit versions seem to just utilize the additional ram.
AutoCAD and Revit run mostly on a single core, so multiple cores/threads don't really help, but I think a real workstation does enough stuff that at least a quad core is beneficial. I do things sometimes that slow down my dual core workstation at work but don't slow down my quad core machine at home.